Dec. 1, 1885.] 



♦ KNOWLEDGE ♦ 



49 



our spectrum to its lowest terms ; and tliat in reality 

 three colour.^, and three only, in various combinations, go 

 to make up the hundreds of thousands of compound tints 

 and liues which glorify the face of nature. For the present, 

 disre2:ardiug any intermediate shades, we will deal, for 

 simplicity's sake, with the spectrum of the five colours 

 which we hnve specified above ; regardini; them as pure 

 when produced by the projection by a prism of the image 

 of a narrow sun-lit slit. The ett'ect of the mixture cf white 

 light with any of them we shall see immedi*tely. This 

 much premised, we will endeavour to investigate the 

 different ways in which the colours are produced 

 which we see around us. With two of these ways we 

 have already dealt in this stries of papers. Interference 

 was treated of on pp. 319 et seq. and 540 of Yol. VII. of 

 the weekly issue of Knowledge ; and polarization referred 

 to on p. 307 of Vol. VIII., and again on p. 17 in our 

 November number. It is though scarcely necessary to 

 add that the colours due to polarization are never seen 

 under ordinary circumstances, and require special appa- 

 r.iitus to exhibit them. We here propose to treat of 

 reflection and absorption. 



Now our first experiment shall be" made with a tumbler 

 iif cle?-n wr.ter and a little milk. In the outset, if we let 

 a beam of light fall upon the surface of the water at any 

 moderate angle, we know that part of the light will be 

 reflected and part will pass through the water: both 

 transmitted and reflected light being alike colourless. 

 Let us uow, though, add a little milk to the water and 

 let us look down upon it. The reflected light will no 

 longer be colourless; it will exhibit a decided bluish tint 

 — a fact popularly recorded in the slang appellation of 

 "sky-blue" to the watered milk of the respectable 

 English tradesman. From tliis it is obvious that the 

 minute globules of milk diffu.sed throughout the wr.ter 

 reflect the blue rays out of those composing the white 

 light which falls ujjon them, i.nd in some way absorb or 

 suppress the rest. But suppose that, in.stead of looking 

 down on to the surfac? of our milky water, we look 

 through it — what then ? Shall we see the transmitted 

 light of a blue tint ? Not at all. It will, on the con- 

 trary, appeir yellowish. Thus it is evident that under 

 certain circumstances, milk, in a fine state of subdivision, 

 divides white light into yellow and blue light. It is 

 almost needless to add that if we continue to add more 

 and more milk to the water, the blue tinge will give wp,y 

 to a white one ; though it may not be quite so familiar a 

 fact, thr.t the light passing through the mixture will, as 

 the milk increases in proportion, become yellow, then 

 orange, and finall}' red, prior to the opacitj^ of the milk 

 becoming so gTeat as to shut out all light whatever. 



Analogous phenomena are exhibited by wood smoke 

 issuing from a cottage chimney, which, as everyone 

 knows, shows as of a pale bluish white as seen against 

 a dark background of trees, but appears brown against the 

 light of the skj'. It is, however, the sky itself which 

 exhibits this phenomenon on the grandest sciiJe. Space 

 itself is of inky blackness, and the records of aerial 

 voyages show what an awful dark appearance the celestial 

 vault presents when the aeronrAit attains any very con- 

 siderable height. The fact is, as has been so admirably 

 explained by Tyndall, that our atmosphere is filled with 

 innumerable billions of particles so minute that they 

 never descend to the earth's surface at all; snow, hail, 

 wind, and rain being alike inoperative upon them. Now 

 when the sun lights these particles they behave in a 

 fashion cognate to that of those of milk in our little 

 experiment just described. They reflect blue and white 

 — or white and blue— light, and this, seen on the black 



Fig;. 



background of space, we see as the familiar blue of the 

 sky. The nearer we are to the earth's surface, the more 

 we get of the white element and the paler the sky tint. 

 If, however, the sky is clear, and the sun is at any con- 

 siderable height, the yellowish tint produced by the 

 transmission of his rays through the particles of our 

 r^tmosphere is scarcely noticeable save in his immediate 

 neighbourhood. When, though, he approaches the 

 horizon, his light has to pass through a greatly- increased 

 thickness of the atmosphere and 

 its contents, for a reason which 

 a moment's study of Fig. 5 will 

 render apparent. Here E repre- 

 sents the earth, A an observer on 

 its surface, the white ring sur- 

 rouuding the earth indicating 

 the atmosphere. If now the 

 sun, S, is in the zenith of the 

 observer — i.e., right ovt'r his 

 head — its light has only the thickness A B to pene- 

 trate. When, however, the sun is setting, a^ r,t Sj 

 its rays have to travel through the whole thick- 

 ness, A B', of air, w ith the familiar result that £S 

 the Sun descends he first looks yellow, then orange, 

 and ultimately when on the horizon fiery red. The 

 reader who has made the little experiment with milk 

 and water described above will now have no difficult}- in 

 accounting for this .sequence of phenomena for himself. 

 Moreover, a little consideration will show that in the gor- 

 geous and glorious chromatic effects of sunset, both 

 reflection and transmission play their parts. What artists 

 call the " warm " tints for example, the crimson, orange, 

 and j-ellow, have their origin in transmitted light. The 

 violet, greyish, and steel-blue of the higher sky depend 

 more or less upon reflection, while many intermediate 

 tints are produced by a combination of both. It is not 

 liowever only at sunrise and sunset that the effects of our 

 atmosphere in the production of colour e.ve rendered 

 apparent. They become particulurlj' marked in clear 

 weather at any hour of the day when we regard the 

 remote distance. Let us take a ra'Jge of distant moun- 

 tains as a familiar example. We know, as a matter of 

 fact, that they may consist of bare reddish- feldspar, of 

 almost black trsp rock, or may be clothed with a carpet 

 of exquisite verdure; but, owing to the miles of inter- 

 vening air, teeming with its invisible particles, the 

 proper colours of the remote hills are shrouded and 

 veiled, and we see them as of a deep and almost aerial 

 blue cr purple. Light and shr.de, as such, almost disap- 

 pear, and the summits, peaks, and ridges stand out r,s 

 though composed of the crystallised material of a cloud. 

 In the middle distance of the landscape, where, of course, 

 the thickness of the veil of light-scattering particles is 

 notably less, the blue atmospheric tints blend with the 

 proper colours of objects, and tend to reduce their lumi- 

 nosity and impart a greyness to them ; while as we 

 approach the immediate foreground the local colour of 

 objects becomes more r,nd more pronounced, until they 

 become, practically, pure immediately under the eye of 

 the ob-erver. 



The vast imjiortance of a knowledge of these rudi. 

 mentarj' facts to all engaged in the art of landscape- 

 painting, whether in water-colours or in oils, as amateurs 

 or professionals, can scarcely be over-estimated. Upon 

 such knowledge depends the power of rendering the 

 varying aspects of any given view with a natural effect. 

 The beginner who neglects to represent the charming 

 influences of the atmospheric trausmission and reflection 

 of light, must perforce fail to produce anything approach; 



